In recent years, advances in audio recording and reproduction have facilitated the development of immersive “surround sound,” in which audio is played back from multiple speakers that surround the listener. Surround-sound systems for home use, for example, include arrangements known as “5.1” and “7.1,” in which audio is recorded for playback over either five or seven channels (three speakers in front of the listener and additional speakers at the sides and possibly behind or above the listener) plus a sub-woofer.
On the other hand, large numbers of users today listen to music and other audio content through stereo headphones, typically via mobile audio players and smartphones. Multi-channel surround recordings are generally down-mixed from 5.1 or 7.1 channels to two channels for this purpose, and the listener therefore loses much of the immersive audio experience that the surround recording is able to provide.
Various techniques for down-mixing multi-channel sound to stereo have been described in the patent literature. For example, U.S. Pat. No. 5,742,689 describes a method for processing multi-channel audio signals, wherein each channel corresponding to a loudspeaker placed in a particular location in a room, in such a way as to create, over headphones, the sensation of multiple “phantom” loudspeakers placed throughout the room. Head Related Transfer Functions (HRTFs) are chosen according to the elevation and azimuth of each intended loudspeaker relative to the listener. Each channel is filtered with an HRTF such that when combined into left and right channels and played over headphones, the listener senses that the sound is actually produced by phantom loudspeakers placed throughout the “virtual” room.
As another example, U.S. Pat. No. 6,421,446 describes apparatus for creating 3D audio imaging over headphones using binaural synthesis including elevation. The apparent location of sound signals as perceived by a person listening to the sound signals over headphones can be positioned or moved in azimuth, elevation and range by a range control block and a location control block. Several range control blocks and location control blocks can be provided depending on the number of input sound signals to be positioned or moved.